The turbines of gas turbine engines operate with gas temperatures on the order of 1650.degree. C. Such elevated temperature limits the allowable stress of various turbine parts, and produces deterioration which reduces the time between maintenance and replacement. The area between the first and second turbine rotor stages experiences high temperatures and is vulnerable By recognizing the temperature limits of the rotating structure we are able to maintain that temperature. Hot gas tends to bypass the stationary vanes adjacent to the first and second rotor disks. Labyrinth seals are used to restrict this leakage. Cooling air is frequently injected into this area to further deter gas path flow from contacting highly stressed parts. U.S. Pat. No. 4,869,640 shows such apparatus and further includes a restricted flow area between the main gas flowpath and the inlet ahead of the labyrinth seal.
Also requiring cooling airflow are the second stage vanes and the second stage turbine outer air seals around the periphery of the turbine. The cooling air is obtained from a compressor stage of high enough pressure to produce the necessary flow. Selection of a higher than necessary stage increases parasitic power loss because of the increased compression. It further supplies hotter air because of this compression.
The maximum need for cooling air is at maximum power which is equivalent to the maximum rotor speed. With the percentage airflow established for this condition, excess air flows at lower operating speeds and temperatures including cruise condition. All of the bypassing cooling air leads to reduced engine and turbine component efficiency. Excess cooling air beyond that required is therefore not desirable since it increases the thrust specific fuel consumption.
It is generally not sufficient to supply only an amount of cooling air to protect the 1-2 seal area with new seals. Additional air must be supplied to handle a worn seal condition. While this flow is only required at high power when the seals are worn, once the design is established this air flow exists at all times.
Of the three cooling air flows discussed above the cooling air to the labyrinth seal area is exposed to the highest gas pressure. Therefore, the differential pressure between the supply chamber and the injection point is less here than it is for the other cooling air flow. Accordingly, introduction of the cooling air into the labyrinth seal area is sensitive to achieve the efficacy of operation.
In the seal area between the first and second rotor stages a seal runner is secured to the turbine disks. This seal runner tends to operate hotter than the disks and therefore places a stress on the disk because of the differential expansion. In the transient from idle to takeoff power the runner heats faster than the disk and therefore the stress level peaks. This leads to low cycle fatigue damage.